1. Technical Field of the Invention
The present invention relates to marine geophysics using electromagnetic detection of buried geological formations. The invention is particularly useful for determining whether a prospective petroleum reservoir indicated in seismics is oil-bearing, and also desirably the horizontal extension of the petroleum reservoir, and determining a range for some of the electrical properties of the petroleum reservoir. The depth, the extension and particularly the electrical properties may provide important information about whether a volume of oil in the reservoir may be distinguished from ubiquitous pore water that is generally present in most porous subterranean rocks.
More specifically, the invention relates to a method for generating very long-wavelength electromagnetic signals under the sea, and detecting electromagnetic waves under the sea, some of which waves have traveled downward, along and upward through geological layers under the sea, as indicated in FIG. 1a. Such very long-wavelength electromagnetic waves for use in the present invention are similar to radio waves, but are of much longer wavelength. Electromagnetic waves are quite severely attenuated in the sea and in the ground due to the electrical resistivity of the rocks with more or less saline water. The attenuation is most severe for higher frequencies. But given a strong electromagnetic source and a very sensitive receiver, and using a low frequency, a signal having traveled through seawater and the ground may be detected at the receiver. Generally sedimentary layers may form an overburden over a deeply buried porous geological layer being a prospective hydrocarbon reservoir. Some of the electromagnetic waves have been reflected by the prospective hydrocarbon reservoir, and some of the waves may have been refracted along the prospective hydrocarbon reservoir. A small proportion of the reflected or refracted electromagnetic energy will reach back to the seafloor in the form of electromagnetic waves, and be measurable with electromagnetic antennae.
2. Description of the Related Art
Several geophysical and direct methods may be used for detecting the presence of a petroleum reservoir. The methods applied may be used in different sequences, depending on the amount of knowledge acquired from previous exploration steps. Magnetometry may be used to determine and map depths to the bedrock below a sedimentary basin, and is rapidly and cheaply measured. Gravity measurements may delineate volcanic stocks and sills that will have a positive gravity anomaly, and salt dome features will display a negative anomaly in gravity profiles and maps. A petroleum reservoir may display a negative gravity anomaly due to the fact that the petroleum fluids are of less density compared to the water that they displace, but such features are usually not of significant size to be directly detected, but may constitute a significantly measurable difference during production of the field. Gravity measurements are also rather rapidly acquired, but rather more time-consuming as compared to magnetometry.                Electromagnetic prospecting uses electromagnetic signals of a wavelength sufficient to penetrate the geological layers under the sea. Detecting the electromagnetic waves may take place either at the seafloor or in the sea water.        
Such electromagnetic prospecting may be used to delineate some geological layers of higher or lower resistivity than their surrounding geological formations. A transmitter antenna is used in the sea for transmitting electromagnetic waves that propagate through the sea and the geological formations. A small proportion of refracted and reflected electromagnetic energy will reach back to the seafloor and be detectable. The detected signals are analysed to indicate petroleum-bearing formations. FIG. 1a indicates such electromagnetic prospecting.                Seismic prospecting utilizes low-frequency sound waves from a seismic source, the waves propagating through the sea and the ground to a seismic receiver. The velocity of seismic waves depend on the density and other mechanical properties of the rocks they propagate through, and the propagation mode of the wave, either as a compression or “p”-wave having the particle motion along the line of seismic energy propagation, or as a transversal, shear or “s”-wave having its particle motion normal to the line of seismic energy propagation. Marine seismics requires a dedicated seismic source and a highly sensitive array of seismic receivers, usually in the form of one or more towed seismic streamers with hydrophones or seabed cables with hydrophones and geophones, and is a far more time-consuming and expensive process compared to gravity. Marine seismics may provide high-resolution reflection seismic profiles that may be processed to show sections of geological structures indicating potentially petroleum bearing petroleum traps, like a porous sand formation in an antiform and covered by an impermeable sedimentary layer, or a porous sand formation vertically offset by a fault. Such delineated potential petroleum-bearing formations however, may rarely be distinguished on the basis of their seismic velocities, because the density and thus the seismic velocity of an oil-bearing formation is only slightly less than the seismic velocity of the same formation being water-filled. However, when having found a potential petroleum bearing formation in the seismic profiles, electromagnetic prospecting may be used to determine some electrical properties of the formation, indicating the presence of water or petroleum, as will be described below.        Drilling is the ultimate and most expensive method to provide geological information about a prospective reservoir. Based on gravity, electromagnetic and seismic exploration and general geological information, an evaluation of the potential field is made. The positions of first exploration or “wildcat” holes are determined and drilled when some or all of the above less expensive methods like gravity and seismics indicate the presence of a petroleum reservoir. Then, if positive results are obtained, production wells are drilled. To find a reasonable indication of the horizontal extension of a reservoir, so-called appraisal or delineating wells may be drilled.        
A major practical problem in marine electromagnetic geophysics is the fact that the sea is conductive, having a conductivity of about 0.3 Ohm-meter due to its salinity. The conductivity incurs significant signal attenuation as the electromagnetic waves propagate through the conductive saline water. Also a major proportion of the rocks from the seafloor and down through all the overburden are more or less conductive, having a conductivity that may vary from 0.3 for generally seawater-wet unconsolidated porous seafloor sediments, to 10 Ohm-meter for more consolidated sediments containing less salt and less ion mobility. However, the electrical properties of a petroleum-bearing rock are significantly different from a saline water-bearing rock. A petroleum bearing sandstone may have a conductivity of about 20-300 Ohm-meter. An deep waters, Ellingsrud et al. in U.S. Pat. No. 6,717,411 have used a transmitter in the form of a towed horizontally arranged dipole electrode pair of 100-1000 m separation and using a 1 Hz alternating current. The wavelength/of the transmission is indicated to be in the range                0.1s<=I<=5s and more preferably        0.5s<=I<=2swhere l is the wavelength of the transmission through the overburden of thickness s. In the example described, the thickness s is 800 m, indicating        80 m<=l<=4000 m, more preferably        400 m<=l<=1600 m.        
The sea depth used in Ellingsruds examples is 1000 m, and the resistivity of the overburden is 0.7 Ohm meter. For wavelengths through the overburden as preferably indicated                80 m<=l<=4000 m, more preferably        400 m<=l<=1600 m, this indicates frequency ranges of        1100 Hz<=f<=0.44 Hz, more preferably        44 Hz<=f<=2.7 Hz.        
The preferred wavelengths indicated by Ellingsrud do not correspond with the indicated transmission frequency range indicated:                1 kHz <=f<=0.01 Hz, more preferably        20 Hz <=f<=0.1 Hz, for example        1 Hz.        
The actually used frequency in Ellingsrud's example is 1 Hz, giving an actual wavelength of 421 m if the resistivity of the overburden is 0.7 Ohm meter.
When towing the transmitter antenna near the seafloor at a sea depth of 1000 m like in Ellingsrud's example, due to the conductivity of the sea water, the air wave poses no significant problem. The sea depths relevant to our present invention may be about 50 to about 350 meters, far shallower than in the above-mentioned US Patent. The depth may even be as shallow as 20 meters or even 10 meters. The air wave is believed to be a significant problem when using a frequency of about 0.5 Hz, please see FIG. 4F, in which there is an insignificant deviation in the normalized curve for a petroleum-bearing reservoir when measuring at a water depth of 128 m. The thickness of the overburden may be between 500 and 3000 meters in the present invention. One may also consider using the present method for verifying the presence of shallow gas as methane or so-called gas hydrates found using shallow seismics at depths shallower than 500 meters. Such gas hydrates may be indicated in the shallow seismics in that their seismic reflection contour follows more or less the contour of the sea floor, but may be verified using electromagnetic methods to indicate a higher resistivity.
One disadvantage of the known art is the use of sine wave pulses, in which the wave is a continuous wave, being difficult to maintain when produced in marine electrical generators that in practice shall be more or less short-circuited through transmitter antennas in the sea. A simpler signal source is sought in the present invention.